<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <script src="../js/vue.js"></script>
  </head>
  <body>
    <!--  
            监视属性watch:
                1.当被监视的属性变化时,回调函数自动调用,进行相关操作
                2.监视的属性必须存在,才能进行监视!!
                3.监视的两种写法:
                    (1).new Vue时传入watch配置
                    (2).通过vm.$watch监视

      -->

    <div id="root">
      <h2>今天天气很{{info}}</h2>
      <button @click="changeWeather">切换天气</button>
    </div>
    <script>
      Vue.config.productionTip = false;

      const vm = new Vue({
        el: "#root",
        data: {
          isHot: true,
        },
        computed: {
          info() {
            return this.isHot ? "炎热" : "凉爽";
          },
        },
        methods: {
          changeWeather() {
            this.isHot = !this.isHot;
          },
        },
        /*
        watch: {
          isHot: {
            //immediate: true,//初始化时 让handler调用一下
            //handler什么时候调用?当isHot发生改变时。
            handler(newValue, oldValue) {
              console.log("isHot被修改了", newValue, oldValue);
            },
          },
        },
        */
      });
      vm.$watch("isHot", {
        //immediate: true,//初始化时 让handler调用一下
        //handler什么时候调用?当isHot发生改变时。
        handler(newValue, oldValue) {
          console.log("isHot被修改了", newValue, oldValue);
        },
      });
    </script>
  </body>
</html>
